Motorized power steering apparatus

ABSTRACT

A motorized power steering apparatus for a vehicle has a torque sensor which generates an output voltage proportional to a steering torque exerted by the driver of the vehicle and a supply voltage. A motor controller controls the output torque of a drive motor which generates an auxiliary steering force corresponding to the output voltage of the torque sensor. The supply voltage for the torque sensor is decreased as the vehicle speed increases, so that the output torque of the motor for a given steering torque decreases as the vehicle speed increases.

BACKGROUND OF THE INVENTION

This invention relates to a motorized power steering apparatus forautomobiles or other vehicles. In the past, power steering apparatusesfor automobiles were usually driven by hydraulic power generated by apump. However, as hydraulic systems are bulky and heavy, in recentyears, there has been a trend towards the use of electric motors toprovide the drive force for power steering. A power steering apparatuswhich employs an electric motor to generate an auxiliary torque toassist the steering of the vehicle is referred to as a motorized powersteering apparatus.

In a motorized power steering apparatus, a torque sensor measures thesteering torque applied by the driver to a steering wheel. An electricmotor which is connected to a suitable portion of the steering gear isthen controlled in accordance with the measured torque to impart anauxiliary steering force to the steering gear. The auxiliary steeringforce lessens the force which need be applied to the steering wheel bythe driver.

Generally, as the speed of a vehicle increases, the resistance betweenthe road surface and the tires decreases, so the force required forsteering the vehicle also decreases. If the auxiliary steering forcewhich is generated by the electric motor were the same at high and lowspeeds, the resistance to steering felt by the driver would become toolight at high speeds. If the driver exerted a sudden torque on thesteering wheel at a high vehicle speed, the wheels of the vehicle couldturn too sharply, possibly upsetting the stability of the vehicle.

Therefore, in a motorized power steering apparatus, the electric motoris controlled so that the torque which it outputs in response to a givensteering torque is lower at high vehicle speeds than at low vehiclespeeds. In many conventional power steering apparatuses, the motor iscontrolled by a microcomputer equipped with an internal memory in whichis stored a table of data relating the motor output to the steeringtorque for all vehicle speeds. When the microcomputer receives signalsindicating the vehicle speed and the steering torque applied by thedriver, based on the data in the memory, it calculates the appropriateamount of torque to be generated by the motor and controls the motorcurrent accordingly.

In order to perform fine, continuous control of the electric motor overthe entire speed range of the vehicle, a large amount of data isnecessary. The internal memory of the microcomputer must have a largecapacity to accommodate all the data, and it therefore tends to beexpensive. Taking everything into consideration, conventional motorizedpower steering apparatuses which employ a microcomputer are complicated,unreliable, and costly.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a motorized powersteering apparatus which can perform fine control of the auxiliarysteering force which it generates without employing a microcomputer.

It is another object of the present invention to provide a motorizedpower steering apparatus which is reliable, simple in structure, andinexpensive.

A motorized power steering apparatus in accordance with the presentinvention has a torque sensor for generating an output voltageproportional to a supply voltage and the steering torque exerted by adriver of the vehicle. A voltage supply supplies the torque sensor witha supply voltage which is inversely proportional to the vehicle speed.An electric motor for applying an auxiliary steering force to a steeringgear is controlled by a motor controller so as to generate a torqueproportional to the output voltage of the torque sensor.

As the vehicle speed increases, the supply voltage which is supplied tothe torque sensor by the voltage supply decreases. The output voltage ofthe torque sensor for a given steering torque therefore decreases withincreasing vehicle speed, and the output of the motor accordinglydecreases. As a result, the motor is prevented from generating too greata torque at high vehicle speeds.

Any device capable of generating a voltage corresponding to the steeringtorque can be employed as the torque sensor. In a preferred embodiment,the torque sensor comprises a torque-displacement converter whichproduces a twisting displacement which is proportional to the steeringtorque imparted to a steering wheel by the driver, and a potentiometerhaving a wiper arm which is moved by the displacement of thetorque-displacement converter. The output voltage of the torque sensoris the voltage at the wiper arm of the potentiometer.

Preferably, the torque sensor 3 has a right turn potentiometer whichgenerates an output voltage proportional to the steering torque onlywhen the steering torque is such as to steer the vehicle to the rightand a separate left turn potentiometer which generates an output voltageproportional to the steering torque only when the steering torque issuch as to steer the vehicle to the left. The motor controller controlsthe voltage which is applied to the motor so that the output torque ofthe motor is proportional to the output voltage of only one of thepotentiometers at a given time. The motor controller preferably includesa logic circuit which prevents the motor from operating when both of thepotentiometers simultaneously generate an output voltage of above aprescribed level.

There are no restrictions of the structure of the voltage supply. In apreferred embodiment, it comprises a vehicle speed sensor whichgenerates an electrical output having a frequency proportional to thespeed of the vehicle, and a frequency-voltage converter which generatesan output voltage which is inversely proportional to the frequency ofthe output signal of the speed sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an embodiment of a motorized power steeringapparatus according to the present invention.

FIG. 2 is a circuit diagram of the control unit of the embodiment ofFIG. 1.

FIG. 3 is a graph showing the output of the torque sensor as a functionof the steering torque applied to the steering wheel by the driver ofthe vehicle.

FIG. 4 is a graph of the output of the frequency-voltage converter as afunction of the speed of the vehicle.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of a motorized power steering apparatus accordingto the present invention will now be described while referring to theaccompanying drawings. As shown in FIG. 1, which is a block diagram ofthis embodiment, a steering wheel 1 of an automotive vehicle is mountedon the upper end of a first steering shaft 2a. The lower end of thefirst steering shaft 2a is connected to the upper end of a secondsteering shaft 2b by a torque sensor 3. The torque sensor 3 includes atorque-displacement converter 3a which produces a displacement which isproportional to the steering torque applied to the steering wheel 1 bythe driver of the vehicle. Any conventional torque-displacementconverter can be employed, such as the torque-displacement converterdisclosed in U.S. Pat. No. 4,666,010 in which a torsion bar whichconnects two steering shafts undergoes angular displacement due totwisting which is proportional to the steering torque applied to asteering wheel. The torque sensor 3 also includes a right turnpotentiometer 3d and a left turn potentiometer 3e. Each potentiometerhas a wiper arm which is moved along a resistive element by thedisplacement of the torque-displacement converter 3a. The voltage at thewiper arm of each potentiometer constitutes an output voltage of thetorque sensor 3 and is provided to a control unit 9 as an indication ofthe steering torque to the right or to the left, respectively.

The lower end of the second steering shaft 2b is connected to the upperend of a third steering shaft 2c by a first universal joint 4a, and thelower end of the third steering shaft 2c is connected to the upper endof a drive shaft 5 by a second universal joint 4b. A pinion gear 5a isformed on the lower end of the drive shaft 5. The pinion gear 5a mesheswith a rack 6 of the steering gear of the vehicle.

An auxiliary torque can be applied to the drive shaft 5 by an electricmotor 7 such as a shunt-wound or permanent magnet DC motor. The motor 7is connected to the drive shaft 5 by a reduction gear 8 which reducesthe rotational speed of the motor 7. The operation of the motor 7 iscontrolled by the control unit 9, which provides the motor 7 with apulse width modulated drive signal.

The control unit 9 is powered by the vehicle battery 10, which typicallyis a 12-volt battery. Portions of the control unit 9 are connecteddirectly to the battery 10, while other portions are connected to thebattery 10 via a key switch 11. A vehicle speed sensor 12 generates anoutput signal having a frequency which is proportional to the vehiclespeed. This output signal is supplied to the control unit 9.

FIG. 2 is a circuit diagram of the control unit 9 of FIG. 1. The controlunit 9 constitutes a self-excited pulse width modulation circuit withfeedback from the motor 7. A first comparator 911 has a positive inputterminal which is connected to the wiper arm of the right turnpotentiometer 3d and a negative input terminal which is connected to thejunction of two series resistors 924 and 925. A second comparator 912has a positive input terminal which is connected to the wiper arm of theright turn potentiometer 3d and a negative input terminal which isconnected to ground through a resistor 942. A third comparator 913 has apositive input terminal which is connected to the wiper arm of the leftturn potentiometer 3e and a negative input terminal which is connectedto the junction of two series resistors 926 and 927. A fourth comparator914 has a positive input terminal which is connected to the wiper arm ofthe left turn potentiometer 3e and a negative input terminal which isgrounded through a resistor 945.

Resistors 924 and 925 are connected in series between the outputterminal of a frequency-voltage converter 950 and ground. Resistors 926and 927 ar likewise connected in series between the output terminal ofthe frequency-voltage converter 950 and ground. A terminal at one end ofthe resistive element of each potentiometer 3d and 3e is connected tothe output terminal of the frequency-voltage converter 950, while theterminal at the other end of the resistive element is grounded.

The output terminals of the first comparator 911 and the thirdcomparator 913 are connected to the output terminal of a 5-volt voltageregulator 902 by pull-up resistors 928 and 929, respectively. The outputterminals of the second comparator 912 and the fourth comparator 914 areconnected to the output terminal of a 26-volt power supply 901 bypull-up resistors 930 and 931, respectively.

The output terminal of the first comparator 911 is also connected to theinput terminal of a first inverter 916 and to one of the input terminalsof a first AND gate 918. The output terminal of the third comparator 913is connected to the input terminal of a second inverter 917 and to oneof the input terminals of a second AND gate 919. The output terminal ofthe first inverter 916 is connected to the other input terminal of thesecond AND gate 919, and the output terminal of the second inverter 917is connected to the other input terminal of the first AND gate 918.

The electric motor 7 is driven by first through fourth power MOSFET's920-923. The gate of the first MOSFET 920 is connected to the outputterminal of the second comparator 912, its drain is connected to thebattery 10, and its source is connected to a first terminal 7a of theelectric motor 7. The gate of the second MOSFET 921 is connected to theoutput terminal of the first AND gate 918, its drain is connected to asecond terminal 7b of the motor 7, and its source is grounded. The gateof the third MOSFET 922 is connected to the output terminal of thefourth comparator 914, its drain is connected to the battery 10, and itssource is connected to the second terminal 7b of the electric motor 7.The gate of the fourth MOSFET 923 is connected to the output terminal ofthe second AND gate 919, its drain is connected to the first terminal 7aof the electric motor 7, and its source is grounded.

The first terminal 7a of the motor 7 receives a positive input voltagewhen the motor 7 is being driven so as to steer the wheels of thevehicle for a right turn, and the second terminal 7b receives a positiveinput voltage during a left turn.

A resistor 941 is connected between the first terminal 7a of the motor 7and resistor 942, and a capacitor 943 is connected from the junction ofresistors 941 and 942 to ground. Similarly, a resistor 944 is connectedbetween the second terminal 7b of the motor 7 and resistor 945, and acapacitor 946 is connected from the junction of resistors 944 and 945 toground. Capacitors 943 and 946 determine the frequency of self-excitedoscillation of the control unit. Via resistors 941 and 944, the secondand fourth comparators 912 and 914 receive feedback signals indicatingthe motor voltage.

The power supply 901 and the voltage regulator 902 are connected to thebattery 1 by the key switch 11. The above-mentioned frequency-voltageconverter 950 receives the output signal of the vehicle speed sensor 12and generates an output voltage which is inversely proportional to thefrequency of the speed signal. The output voltage of thefrequency-voltage converter 950 is provided to the potentiometers 3d and3e and resistors 924 and 926 as a positive supply voltage.

FIG. 3 illustrates the output characteristics of the torque sensor 3 asa function of the steering torque applied to the steering wheel 1 by thedriver. When no steering torque is applied to the steering wheel 1, theoutput of both potentiometers 3d and 3e is zero volts. When a rightwardtorque is applied to the steering wheel 1, the output of the right turnpotentiometer 3d increases linearly with increasing torque until thesteering torque reaches a value of approximately 0.8 kgf-m, at which theoutput voltage saturates at a voltage V_(SR). When the right turnpotentiometer 3d has a non-zero output, the output of the left turnpotentiometer 3e remains at zero volts. Conversely, when a leftwardsteering torque is applied to the steering wheel 1, the output of theright turn potentiometer 3d is zero volts, while the output of the leftturn potentiometer 3e increases linearly with increasing steering torqueuntil the steering torque reaches approximately 0.8 kgf-m, upon whichthe output saturates at a voltage V_(SL). As will be explained below,the levels of V_(SR) and V_(SL) depend upon the vehicle speed.

FIG. 4 is a graph illustrating the output characteristics of thefrequency-voltage converter 950 as a function of the vehicle speed. At alow vehicle speed of up to approximately 5 km/hr (point f in FIG. 4),the frequency-voltage converter 950 generates a constant voltage ofapproximately 5 volts. At a vehicle speed of greater than approximately20 km/hr (point g), the frequency-voltage converter 950 generates aconstant output voltage of approximately 2.5 volts. At a vehicle speedbetween approximately 5 and 20 km/hr, the output voltage of thefrequency-voltage converter 950 linearly decreases as the vehicle speedincreases.

The output voltage of the frequency-voltage converter 950 is supplied tothe potentiometers 3d and 3e as a positive supply voltage. Since theoutput voltage of the frequency-voltage converter 950 varies inaccordance with vehicle speed, the output characteristics of the torquesensor 3 are also dependent on the vehicle speed. As shown in FIG. 3,when the vehicle speed is below that corresponding to point f of FIG. 4(approximately 5 km/hr), the output characteristics of thepotentiometers 3d and 3e are as shown by curves a and a', respectively,which have a steep slope and a high saturation voltage V_(SR)(L) andV_(SL)(L) of approximately 5 volts. When the vehicle speed is above thatcorresponding to point g of FIG. 4 (approximately 20 km/hour), theoutput characteristics of the potentiometers 3d and 3e are shown bycurves b and b', respectively, which have a more gradual slope and alower saturation voltage V_(SR)(H) and V_(SL)(H) of approximately 2.5volts. When the vehicle speed is between 5 and 20 km/hour, the outputcharacteristics of the potentiometers 3d and 3e are described by curveslying somewhere between curves a and b or a' and b'. For example, whenthe vehicle speed is that corresponding to point h of FIG. 4(approximately 12.5 km/hour) midway between points f and g, the outputcharacteristics of the potentiometers 3d and 3e are as shown by curves cand c', respectively, lying midway between curves a and b (or a' andb').

When the unillustrated engine of the vehicle is started and the keyswitch 11 is closed, 12 volts are supplied by the battery 10 to thepower supply 901 and the voltage regulator 902 and the control unit 9begins to operate. It will be assumed that the vehicle is initiallystationary, so the vehicle speed sensor 12 generates a signalcorresponding to a speed of 0 km/hour. As shown in FIG. 4, thefrequency-voltage converter 950 therefore generates a maximum outputvoltage of approximately 5 volts. This output voltage is supplied to thepotentiometers 3d and 3e and to resistors 924 and 926 as a positivesupply voltage. If the driver then applies a steering torque to thesteering wheel 1, the torque sensor 3 generates an output voltagecorresponding to the magnitude and direction of the steering torque asshown in FIG. 3.

When the driver applies a steering torque to the steering wheel 1 toturn the wheels of the vehicle to the right, the voltage of the wiperarm of the right turn potentiometer 3d is input to the positive inputterminals of the first and second comparators 911 and 912. If the outputvoltage of the right turn potentiometer 3d exceeds the voltage V_(N) atthe junction of resistors 924 and 925, the output of the firstcomparator 911 goes high. As no voltage has yet been applied to themotor 7, the voltage at the junction of resistors 941 and 942 is low,and the output of the second comparator 912 goes high. The outputvoltage of the left turn potentiometer 3e is 0 volts, so the outputs ofthe third and fourth comparators 913 and 914 are low. Therefore, theoutput of the first inverter 916 is low and the output of the secondinverter 917 is high, so the output of the first AND gate 918 is high,the output of the second AND gate 919 is low, and the first and secondMOSFET's 920 and 921 are turned on. Therefore, a voltage from thebattery 10 is applied to the motor 7 through the first MOSFET 920, andthe motor 7 begins to conduct. The positive input voltage for the motor7, i.e., the voltage at terminal 7a, is also applied across resistors941 and 942. Therefore, the voltage at the junction of resistors 941 and942 rises exponentially with a time constant determined by capacitor943. When the voltage at the junction of resistors 941 and 942 exceedsthe voltage at the positive input terminal of the second comparator 912,which is the output voltage of the right turn potentiometer 3d, theoutput of the first comparator 911 remains high but the output of thesecond comparator 912 goes low, so the first MOSFET 920 is turned off,and the supply of current to the motor 7 is interrupted. When thisinterruption takes place, the voltage at the junction of resistors 941and 942 falls exponentially with a time constant determined by capacitor943. When the voltage at the junction of resistors 941 and 942 fallsbelow the voltage at the positive input terminal of the secondcomparator 912, the output of the second comparator 912 again goes high,and the first MOSFET 920 is again turned on to drive the motor 7. Inthis manner, the second comparator 912 is repeatedly turned on and off,and a series of pulses are supplied to the motor 7 as a supply voltage.A current which is determined by the supply voltage and the counter emfcorresponding to the rotational speed of the motor 7 flows through themotor 7. The motor 7 generates an auxiliary torque to turn the wheels ofthe vehicle to the right, and the steering torque which need be exertedby the driver of the vehicle is reduced. The width of the pulses whichare supplied to the motor 7 is dependent on the output voltage of thepotentiometer 3d. The greater the output voltage of the potentiometer3d, the longer is the pulse width.

When the driver exerts a steering torque to turn the vehicle to theleft, the third and fourth comparators 913 and 914 are controlled in amanner similar to that described above with respect to the first andsecond comparators 911 and 912, and a voltage is supplied to the motor 7to steer the wheels of the vehicle to the left.

As shown in FIG. 3, the output voltage of the potentiometers 3d and 3ecorresponding to a given steering torque decreases as the vehicle speedincreases. Since the torque generated by the motor 7 decreases as theoutput voltage of the potentiometers 3d and 3e decreases, it followsthat the auxiliary torque generated by the motor 7 in response to aprescribed steering torque decreases as the vehicle speed decreases.This decrease in auxiliary torque compensates for the decrease in theresistance to steering as the vehicle speed increases. Therefore, thefeel of the steering wheel remains comfortable without becoming toolight at high vehicle speeds, and the safety of the vehicle ismaintained.

The AND gates 918 and 919 prevent the motor 7 from operating unless theoutput of either the first comparator 911 or the third comparator 913 ishigh. This state occurs when the output voltage of the right turnpotentiometer 3d exceeds the voltage V_(N) at the junction of resistors924 and 925, or when the output voltage of the left turn potentiometer3e exceeds the voltage V_(N) at the junction of resistors 926 and 927.If the driver exerts only a very low torque of less than T1 on thesteering wheel 1, the outputs of the potentiometers 3d and 3e will notexceed V_(N), so the motor 7 will not be turned on. Accordingly, thereis a dead band of steering torque on either side of a neutral torque inwhich power steering is not performed. The magnitude of the dead band,i.e., the magnitude of T1, depends on the value of V_(N) and on theoutput characteristics of the potentiometers 3d and 3e. In the presentembodiment, V_(N) is a function of the output voltage of thefrequency-voltage converter 950, and so V_(N) varies with the vehiclespeed. As shown in FIG. 3, at a low vehicle speed below thatcorresponding to point f of FIG. 4, V_(N) has a maximum value ofV_(N)(L), and at a vehicle speed above that corresponding to point g ofFIG. 4, it has a minimum value of V_(N)(H). When the vehicle speed issomewhere between that corresponding to points f and g, V_(N) has alevel somewhere between V_(N)(L) and V_(N)(H). Thus, V_(N) decreases asthe vehicle speed increases. However, as V_(N) decreases, the slopes ofthe curves defining the output characteristics of the potentiometersalso decrease, so T1 and the magnitude of the dead band remain constantat all vehicle speeds.

Normally, only one of the potentiometers generates an output voltage ata time. However, if the potentiometers should malfunction (due, forexample, to noise generated by the torque sensor itself, bad contacts,broken wires, or short circuits) and both generate an output voltage ofgreater than V_(N) at the same time, the outputs of both AND gates 918and 919 will go low and prevent MOSFET'S 921 and 923 from conducting.Therefore, the motor 7 will not be able to operate, and there will be nopossibility of the motor 7 exerting an auxiliary torque in a directionopposite to the direction in which the driver wishes to steer thevehicle. Although the steering will feel heavy when the motor 7 does notoperate, as the steering wheel 1 is mechanically linked to the rack 6,the driver will still be able to safely steer the vehicle.

A motorized power steering apparatus in accordance with the presentinvention can perform fine control of the auxiliary torque which itgenerates over the entire speed range of a vehicle without employing amicrocomputer. It is therefore inexpensive, reliable, and easy tomanufacture.

What is claimed is:
 1. A motorized power steering apparatus for avehicle, comprising:torque sensing means for generating an outputvoltage proportional to both a variable supply voltage applies theretoand a steering torque exerted by a driver of the vehicle, said torquesensing means including resistive voltage dividing means connectedbetween supply voltage terminal means and ground; a motor (7) forgenerating an auxiliary steering force; motor control means (9) forcontrolling an output torque of the motor in accordance with the outputvoltage of the torque sensing means; and speed responsive voltage supplymeans (12, 950) coupled to said supply voltage terminal means forproviding the torque sensing means with a variable supply voltage whichdecreases as the vehicle speed increases.
 2. A power steering apparatusas claimed in claim 1, wherein the voltage supply means comprises:avehicle speed sensor (12) which generates a vehicle speed signal havinga frequency proportional to the vehicle speed; and a frequency-voltageconverter (950) which generates an output voltage which decreases as thefrequency of the vehicle speed signal increases and which is supplied tothe torque sensing means as the variable supply voltage.
 3. A powersteering apparatus as claimed in claim 1, wherein the torque sensingmeans comprises a right turn potentiometer (3d) for generating an outputvoltage corresponding to a steering torque exerted by a driver of thevehicle only when the steering torque is in a direction to turn thevehicle to the right and a left turn potentiometer (3e) for generatingan output voltage corresponding to a steering torque exerted by thedriver only when the steering torque is in a direction to turn thevehicle to the left.
 4. A power steering apparatus as claimed in claim3, wherein:the torque sensing means includes a torque-displacementconverter (3a) for generating a displacement proportional to a steeringtorque exerted thereon; and each of the potentiometers has an inputterminal which is connected to the voltage supply means, a resistiveelement constituting said voltage dividing means, and a wiper arm whichis linked to the torque-displacement converter so as to be moved alongthe resistive element by the displacement of the torque-displacementconverter, the output voltage of each potentiometer being the voltage atits wiper arm.
 5. A power steering apparatus as claimed in claim 3,further comprising means (918, 919) for preventing the motor fromoperating when both of the potentiometers generate an output voltageabove a prescribed level at the same time.
 6. A power steering apparatusas claimed in claim 1, further comprising limiting means for preventingthe motor from being driven when the output voltage of the torquesensing means is below a prescribed voltage which decreases as thevehicle speed increases.
 7. A power steering apparatus as claimed inclaim 1, wherein the motor control means comprises a self-excited pulsewidth modulation circuit with feedback from the motor for supplying themotor with drive pulses having a pulse width which is modulated by theoutput voltage of the torque sensing means.
 8. A power steeringapparatus as claimed in claim 7, wherein the pulse width modulationcircuit comprises:a bridge circuit consisting of a plurality ofswitching elements, (920-923) the motor being connected across thebridge circuit; a capacitor and a resistor connected in parallel betweenthe motor and ground; and a comparator having a first input terminal towhich the output voltage of the torque sensing means is applied and asecond input terminal connected to the junction of the capacitor and theresistor, the output voltage of the comparator being supplied to a gateone of the switching elements of the bridge circuit.